Printed circuit board assemblies are used in the manufacture of a wide variety of electrical and electronic devices. The printed circuit board assemblies typically include a printed circuit board which has a core made of a dielectric material with a metal circuitry layer formed on one surface of the core. The printed circuit board assemblies also include leaded electronic components which are mounted on the upper surface of the printed circuit board, that is, the side opposite the surface on which the metal circuitry layer is formed, with the leads of the leaded components extending through the core to solder pads formed as part of the metal circuitry. Leadless electronic components are also mounted on the surface of the printed circuit board having the circuitry, with the terminals of the leadless components being in contact with solder pads of the metal circuitry.
In order to make the printed circuit board assemblies function satisfactorily it is necessary that electrical connections be made between the leads and terminals of the electronic components and the metal circuitry. The most effective method of making the required electrical connections is to solder the leads and terminals of the components to the circuitry. This can be done by individually hand soldering each lead and terminal, but this is highly impractical, especially in the commercial production of printed circuit board assemblies which may have literally hundreds of individual electrical connections on a single printed circuit board assembly.
Wave soldering is a technique which has been developed for mass soldering of printed circuit board assemblies in which the printed circuit board assemblies are soldered by passing the assembly with the metal circuitry side down through a standing wave of molten solder. The molten solder, under ideal conditions, should solder 100 percent of the leads and terminals to the circuitry. It has been found, however, that even under normally satisfactory operating conditions, considerable difficulties are often encountered in obtaining 100 percent soldering of the joints. The most common problem which occurs is that during wave soldering a number of connection points are not soldered. Part of the reasons for the problems is that there are a large number of process variables involved in wave soldering, many of which are interrelated and which can significantly and adversely affect solderability. These variables include the alloy composition of the solder, and more specifically, the presence and types of impurities in the solder composition, the temperature of the solder bath, the type and amount of flux utilized in the soldering process, the dynamics of the solder wave, the rate of travel of the printed circuit board assembly through the soldered wave, the placement of the electronic components and solder pads relative to the molten solder wave and other similar variables. In addition to the above noted variables, it is often found that there are process variations across the width of the solder wave which cause differences in the quality of the resulting solder connections in the cross machine direction.
Numerous test methods have heretofore been suggested to measure the process variables encountered in wave soldering. These test methods include chemical analysis of the solder composition to determine the alloy composition of the solder and the presence and amount of impurities and measurement of the surface tension and the temperature of the molten solder in the wave. Other variables which are regularly measured include, for example, the amount of flux and oil in the solder wave. The above noted tests at best, however, only give an indirect indication of whether satisfactory soldering can be obtained with a particular solder system.
A method which is commonly employed to directly evaluate the soldering properties of a given wave soldering system is to use unsoldered printed circuit board assemblies as test pieces and attempt to solder the printed circuit board assemblies in the wave soldering system to determine the soldering properties of the soldering system. The printed circuit board assemblies which are test soldered are examined to determine the relative number of the total connections that are correctly soldered. The use of printed circuit board assemblies as test pieces provides results which are directly related to the solderability of a given wave soldering system for a particular printed circuit board assembly but has several distinct disadvantages. Initially, it is an inherently expensive test method in that if satisfactory results are not immediately obtained on the first solder test, which rarely occurs, it can result in the waste of numerous high cost circuit board assemblies until the correct combination of soldering conditions is achieved. A further problem with directly testing soldering properties with unsoldered printed circuit board assemblies is that the results which are obtained are at best generally only indicative of the relative number of satisfactory solder joints obtained with a particular combination of soldering parameters. The test soldering of printed circuit board assemblies does not provide quantitative data which is highly desirable in order to establish how stable the molten solder bath system is with regard to conducting wave soldering of numerous printed circuit board assemblies. The quantitative evaluation of the soldering properties of a wave soldering system is extremely important especially when conducting relatively long soldering runs. It has been found that even if the soldering process parameters appear satisfactory on the test runs, it often happens when production scale soldering of printed circuit board assemblies is commenced, that extremely small changes in one or more of the process parameters, such as the solder temperature or the like, can occur which can cause an almost immediate shift in soldering properties and extremely poor soldering results. Accordingly, it is extremely important to have a quantitative evaluation of the relative stability of the wave soldering system so as to be able to predict the long range capability of the wave soldering system for use in commercial production of printed circuit board assemblies.
Printed circuit boards which have the required circuitry but which do not include the electronic components have also been utilized for the purpose of evaluating the soldering properties of wave solder systems. The technique most commonly employed is to pass the printed circuit boards through the wave soldering apparatus in exactly the same manner as printed circuit board assemblies so as to obtain an indication of the solderability properties of the wave soldering system. The use of the printed circuit boards without the electronic components does substantially reduce the test cost. This technique, however, at best again only gives a result of the solderability of the board without components and the data for the printed circuit board have not always been found to be directly correlatable to the results obtained when soldering printed circuit board assemblies. The problem of the lack of correlation when using printed circuit boards is especially acute when the circuit board assembly to thereafter be soldered includes leadless components on the metal circuitry side.
What would be highly desirable would be a test method and apparatus which could evaluate quantitatively the soldering properties of a wave soldering system in a manner which would be both simple to conduct, relatively inexpensive and which would provide quantitative data directly related to the soldering properties of printed circuit board assemblies in the wave soldering system.